Carb CFM Needs

Take 1/2 of the total cubic inches and divide it by 1728 cubic inches (the amount of cubic inches in a cubic yard). This gives you a multiplier that is air displaced per crank revolution.

Multiply this by your max rpm.

Multiply that by your volumetric efficiency. (90% is reasonable).

This is the amount of cfm required at your given max rpm. Remember that just because a carb is rated at a certain cfm, doesn't mean it will flow that much air since you have to take frictional losses into consideration as well.